An integrated circuit package for surface mounting in a hybrid circuit has tinnable areas enabling it to be soldered to corresponding tinnable areas provided on the substrate of the hybrid circuit, where the tinning of said areas consists in adding metal thereto for the subsequent soldering operation. The added metal, commonly called "solder", is generally constituted by a eutectic tine-lead alloy.
As shown in FIG. 1, an integrated circuit package for surface mounting in a hybrid circuit thus includes tinnable areas such as 1 referred to herein as "connection" areas, which areas are situated at the periphery of the bottom face 2 of the package. The package is generally substantially rectangular in shape and of small thickness, and the areas constitute extensions over said bottom face of outside connection tabs as 3 which are otherwise accessible via the side faces of the package. These connections are connected to the chip of the integrated circuit by feedthroughs that terminate inside the package on connection tabs of said chip.
Some packages may also have a tinnable area such as 4 referred to herein as a "heat exchange" area and/or as a "mechanical fixing" area, for providing heat exchange between the integrated circuit and the outside and/or for providing mechanical fixing of the package to the hybrid circuit substrate.
As recalled in FIG. 1, the outside connection tabs are concave in shape where they extend the connection areas, thereby making it possible to establish reserves of solder that extend over the connection areas during the operation of tinning said areas, and making it possible in turn, after soldering to the substrate, to obtain fillets of solder which can be used, in particular, for visual inspection of the outside connections of the package.
Tinning the tinnable areas of an integrated circuit package generally comprises a first step during which a certain quantity of solder, generally a quantity greater than that required, is applied in the liquid state to said tinnable areas by techniques such as dipping, and a subsequent step during which the excess quantity of solder is removed while the solder is still in the liquid state.
Various methods are known for performing said subsequent step.
A first method consists merely in applying a manual jolt to the package. It will nevertheless be understood that this method is difficult for the operator to learn and is difficult to reproduce since it is not automated. In addition, this method does not make it possible to obtain uniform thickness of solder over all of the tinnable areas of the package, together with adequate filling of the reserves in the peripheral connections, since the amount of solder therein is a function of the orientation of the package when jolted.
A second method consists in tilting the package, e.g. by withdrawing the package from a solder bath tangentially to the surface thereof. This method also suffers from the drawback of not making it possible to obtain a uniform thickness of solder on the tinnable areas of the package, and of not adequately filling all of the reserves since filling of the reserves is a function of the orientation given to the package during said tilting operation.
A third method as described, for example, in French Pat. No. 2 570 220, consists in rotating the package as it leaves the bath about an axis perpendicular to its top or bottom faces so as to eliminate the excess quantity of solder by centrifuging. The main drawback of this method is that it is ill-suited to packages provided with both types of tinnable area mentioned above since it does not make it possible to obtain both a thin deposit on the central heat exchange and/or mechanical fixing area close to the axis of rotation and a sufficient quantity of solder in the peripheral reserves of the connection areas, where the effect of centrifugal force is at a maximu.
In order to avoid soldering defects appearing after the package has been soldered on the substrate of a hybrid circuit by remelting the solder applied in this way during the tinning operation, it is essential for the thickness of said solder on said tinnable areas of the package to be as thin and as uniform as possible and for the quantity of solder in said reserves to be sufficient, but only just sufficient.